Increasing the Resol ution in Locating Cel l ular Telephones
The present invention relates to an apparatus and method for improving the resolution with which the geographical location of a moving cellular telephone system mobile station may be determined.
Many proposals have been made for locating equipment such as vehicles by
reference to broadcast telecommunications signals. For example, low resolution
navigation systems are known which rely on a network of dedicated low frequency
transmitters, the mobile station measuring the phase difference between the signals
received from different transmitters. By comparing the signals from the different
transmitters the mobile station position can be established to within say 50 or 100
metres. More accurate satellite-based systems are now available which can give very
accurate position information, typically down to 10 metres or less, but such systems
rely upon the mobile station having a line of sight path to three satellites and this is
not easy to achieve in urban environments. Furthermore, satellite-based global positioning systems require substantial expenditure on equipment even if governments
continue to make the basic satellite broadcasting system available free of charge.
Thus the known global positioning systems require substantial infra-structure
investment and. if they are to provide high accuracy, substantial expenditure on dedicated equipment at the mobile stations.
Other proposals have been made for determining the location of vehicles, for
example that referred to as the "Cursor" system which relies upon detecting signals
broadcast by local public radio transmitters operating on the medium wave band to
calculate the position of a mobile station. High accuracy is claimed (typically 5 metres) and the infra-structure in the form of freely broadcasting transmission stations
is already present. The Cursor system does nevertheless require relatively expensive
dedicated equipment to be provided in each mobile station.
Cellular telephone systems have grown explosively in recent years. Digital
systems are now being introduced, and in Europe some such systems operate in
accordance with the GSM standard. There are alternative standards which system
operators use, for example PCM. and the present invention is applicable to all the
known standards. For the purposes of explanation however the invention will be
described with reference to only the GSM standard. In cellular telephone networks,
individual subscribers carry mobile stations which receive signals from and transmit
signals to a series of base stations. The territory covered by the system is broken
down into a series of cells each of which is served by a series of base stations. As the
mobile station moves from one cell to another, communication signals are handed
over from one base station to another on the basis of the strength of the signals
transmitted by the base stations at the current location of the mobile station. It is
fundamental to the operation of a cellular telephone network that the system knows
the whereabouts of the mobile station, and this is achieved by causing the mobile
stations to transmit signals to the base stations periodically, for example when the
mobile stations are switched on, or when the mobile stations transfer from one cell to
another, or on a simple periodic time basis.
It is known with a GSM system to be possible not only to locate which cell a
particular mobile station is within, but also to identify a sub-area of the cell within
which the mobile station is located. This is relatively easily achieved by reference to
the relative strengths of the signals received by the mobile station from each of the
base stations within its area. Unfortunately, given the structure of the GSM system
the location of a mobile station can only be determined to within about 200 metres.
Thus a system operator can identify the fact that a mobile station is within an area
which is roughly circular and has a diameter of about 200 metres, but cannot improve
the accuracy of the available positional information. In some circumstance
knowledge of the position of a mobile station of such low resolution is useful, for
example to an operator trying to control a fleet of vehicles which is widely spread over a region, but in many other circumstances such accuracy is of little use. In
particular, the precise location of for example a stolen vehicle in which a mobile
telephone is being used cannot be determined.
A security system has been proposed for locating stolen vehicles. In that system, protected vehicles are provided with a transmitter which is activated from a
central location in the event of the carrying vehicle being stolen. That transmitter
generates a signal specific to the vehicle which enables the police to locate the precise
position of the vehicle. The system is however relatively cumbersome in operation,
requiring substantial infra-structure and operational support from the police, and
therefore although it is useful for recovering valuable vehicles it has not been used
widely because of cost considerations.
By the very nature of cellular telephone systems, mobile stations of such
systems are often moving. If the movements of such a mobile station were random,
low resolution knowledge of the movement of the mobile station would not add
significantly to the accuracy of the available positional information. In fact however
movements of a mobile station are not random as almost invariably they follow
relatively well-defined routes, for example roads or railway lines, and those routes are
normally selected on a rational basis so as to minimise the distance between the start
and finish points of a particular journey. It is an object of the present invention to
make use of the characteristics of the motion of a mobile station of a cellular
telephone system so as to improve the resolution with which the geographical location
of such a mobile station may be determined.
According to the present invention, there is provided a method for improving
the resolution with which the geographical location of a moving cellular telephone
system mobile station may be determined, wherein signals broadcast by cell base
stations are monitored periodically by the mobile station to generate mobile station
position data, the position data representing a series of geographical areas in a region
through which the mobile station moves as the signals are monitored, the position data
is compared with map data representing transport routes within the region to generate
a series of subsets of routes, each subset corresponding to routes within a respective
said area, and the subsets are compared to identify the route along which the mobile
station is moving by reference to the differences between the routes identified by
successive subsets in the series.
The present invention relies on the assumption that a moving mobile station
will be travelling along one of a series of predetermined routes, generally those routes being represented by the road network. Given that the GSM cellular system enables
location of a mobile station to within 200 metres, in regions were there are few roads
it may be that this relatively low resolution positional information will be sufficient to
make it clear that the mobile station is moving along a single road. In regions of high
road density where they may be say fifty routes within a 200 metre diameter area,
most of those routes will be very small and hence, travelling speeds will be low. Thus
knowledge of a vehicles speed enables an accurate prediction of which routes the
vehicle may be on. Journeys generally include portions in which speeds are significant in the context of knowing the whereabouts of a mobile station within 200
metres. Furthermore, journeys are generally such that the mobile station moves in a
logical manner between two points and therefore, once it is known that a mobile
station is moving along a particular road, it is generally possible to accurately predict
subsequent movements of the mobile station, and if appropriate to intercept the
vehicle carrying the mobile station.
Various techniques can be used for assessing the relative probabilities of the
mobile station moving along individual routes represented by the subsets. For
example, the speed at which the mobile station is travelling may be calculated from
the position data, and that calculated speed may be compared with a predicted speed
for each of the routes in the subsets. The predicted speeds for individual routes may
be updated on the basis of information gathered from sources of information about the
flow of traffic such that for example if a particular route was blocked as the result of a
traffic incident that information would be available to improve the accuracy of the
assessment of which route the mobile station might be travelling along.
An alternative or additional approach would rely upon real time predictions of
a probable next location of the mobile station on the basis of the generated position
data. When the signals are next monitored the actual position can then be compared
with the prediction to assess the relative probabilities of a mobile station moving
along the routes represented by the subsets.
Another approach which may be adopted would be to calculate estimates of
the time it will take for the mobile station to travel from each route in the subset to
adjacent routes, the prediction being compared with the time that elapses before that
other route is identified in a subsequent subset.
In a further approach, position data from a number of mobile stations may be
correlated to identify groups of mobile stations which are moving at substantially the
same speed in the same area. That information would make it possible to compare the
correlated data with the capacities of routes identified by the subsets so as to increase
the accuracy with which the position of the mobile stations may be assessed.
The present invention also provides an apparatus for improving the resolution
with which the geographical location of a moving cellular telephone system mobile
station may be determined, wherein the mobile station comprises means for
periodically monitoring signals broadcast by cell base stations, and the system
comprises means for generating mobile station position data from the monitored
signals, the position data representing a series of geographical areas in a region
through which the mobile station moves as the signals are monitored, means for
storing map data representing transport routes within the region, means for comparing
the position data with the map data to generate a series of subsets of routes each of
which corresponds to routes within a respective said area, and means for comparing
the subsets to identify the route along which the mobile station is moving by reference
to the differences between the routes identified by successive subsets of a series.
Embodiments of the present invention will now be described, by way of
example, with reference to the accompanying drawing which schematically illustrates
the manner in which the method according to the present invention may be put into
effect.
Referring to the accompanying drawing, the straight lines represent vectors
based on for example the available OSCAR data, each vector corresponding to a short
section of a road network. The circles represented in broken lines represent areas
within the region of the represented road network within which a mobile station being
tracked is located at any particular time. Each of the circles represents an area having
a diameter of approximately 200 metres. The region within which the represented
road network is located is overlaid by a series of cells of a cellular telephone system and within that region there will be an array of base stations that transmit signals to
any mobile stations within range. In accordance with the known GSM mobile station
location system, the mobile station monitors the strength of the signals received from
the in-range base stations and transmits to the system data which enables the position
of the mobile station to be determined with a reduction represented by the circles.
If we assume that the mobile station is caused to report its position every thirty
seconds, it might be that the mobile station reports its position at these thirty second
intervals such that the mobile station is successively in the areas represented by circles
1, 2, 3 and 4. It is clear in this example that the mobile station is travelling at a
reasonable speed along a relatively straight highway. If it was desired to intercept a
vehicle carrying the mobile station it would be a relatively simple matter to follow the
vehicle on the basis of the information represented by the circles 1 to 4. The series of
circles 1, 5, 6 and 7 represent an alternative route through the road network in which
the vehicle carrying the . mobile station has turned left on to a side road but then
accelerates, the speed of the vehicle being represented by the space between the
centres of adjacent circles. Once again it would be a simple matter to track such a
vehicle.
As a third example, the circles 1 and 8 to 13 represent an alternative route in
which the vehicle carrying the mobile station first turns right, then turns left, and then
terminates its journey somewhere within the area represented by circle 13. It is not
possible to precisely pin-point the location of the vehicle when its journey is
terminated but it is for example clear that the vehicle is located on one of the routes
represented by vectors 14, 15 and 16 and cannot be on the route represented by vector
17. It would therefore be possible to identify the vehicle carrying the mobile station
when it has next moved and to intercept the vehicle on the only road from which the vehicle can reach the general road networks.
Taking the case of the journey represented by circles 1, 5, 6 and 7, the manner
in which the invention operates will be described in further detail. When the mobile
station reports its position as represented by the circle 1 , a list representing a subset of
the vectors describing the road network is generated, that list identifying vectors 18 to
21. When the mobile station is in the area represented by circle 5. a new list of
possible road vectors would be produced identifying vectors 20. 21 and 22. When the
vehicle is in the position represented by area 6, the related list of possible vectors
would be limited to vectors 22, 23 and 24. When the mobile station is in the region represented by area 7, the possible vector list would be limited to vectors 23 and 25.
Thus by comparison of the lists of vectors corresponding to areas 1 and 5, it would be
apparent that the vehicle had turned left given that vector 22 was present in the list
corresponding to area 5. It could also be apparent that the vehicle had not
subsequently turned right on to the road represented by vector 24 given that vector 25
is in the list corresponding to area 7. Thus, assuming the provision of a database
describing all the roads within a region of interest, it is a relatively simple matter to
rapidly scan that database to determine which route the mobile station is travelling
along. Accordingly the present invention proposes the combination of the relatively
low resolution location information available from the GSM system with readily
available map information using iterative analysis of a series of location fixes.
It will be appreciated that when a mobile station stays in the same location it is
difficult to determine exactly which route that mobile station is on. A slow moving
mobile station is also difficult to pin-point. The frequency with which location fixes
are made may however be adjusted to match the particular circumstances by reference
to the speed of travel of the mobile station. For example, a mobile station may be set
up to automatically report its position every say thirty seconds. Alternatively the
mobile station could report its position only when prompted to do so by a signal
transmitted to it over the cellular network. In the latter case it would be a simple
matter for the signals transmitted over the cellular network to be transmitted at a frequency tailored to the severity of the problem of trying to pin-point the mobile
station position. The analysis of the position information can rely on any convenient
techniques. For example the average speed at which the mobile station is travelling
can be calculated from the spacing between successive positions. This information
can be used to determine which routes the mobile station is most probably on. For
example, if the calculated speed is close to the know average speed on a particular
road, or is close to the speed limit on that road, we can be reasonably confident that
the mobile station is moving down that road rather than a road which has a much
lower average speed or speed limit. This approach would enable for example
confusion between a minor road which follows a motorway and the motorway itself to
be eliminated.
Given the provision of a list of roads that the mobile station must be one, the
mobile station speed and direction make it possible to predict in advance where one
would expect the next location fix to be, given that the mobile station is on a
particular road. The next location fix would then closely match the prediction for
some possible roads, produce a less likely match for other roads, or not match the
prediction at all. Using these predictions it is possible to inform the mobile station to
increase or decrease the rate at which it sends in location details. This enables the
system to tune the frequency with which the mobile station reports its position to the
local density of the road network.
A further approach to improving the resolution of the system would be to
calculate for each road section in a list corresponding to one location area the likely
time it would take to get from that road section to another road section not in the list.
By comparing this calculation with the actual time the mobile station takes to get to a
position in which its next location report includes in its area the predicted destination
road section it would be possible to see which prediction matches the outcome most
closely, and which is unlikely (for example having to travel at very high speed to get
from one road section to the other).
A further method of improving resolution would be to look at a whole series
of location fixes from numerous mobile stations centred in a given location. In this
situation where there is a flow of traffic, it can be assumed that mobile stations in the
same traffic flow will be moving at similar speeds. Combining the probable road
section that each mobile station could be on would, in most cases, narrow down the
possible road sections to just one road section.
Data from external traffic monitoring systems (such as TrafficMaster,
Road Watch etc) could be used to determine traffic conditions on certain roads. If the
speed and direction characteristics for a particular mobile station closely matched this
data then one could be relatively sure that the mobile station is on the given road.
The above techniques could be supplemented by alternative techniques to
improve the match between location information and road network information. The
system could also rely upon map information related to more than the simple road
network, for example the rail system. Furthermore the mechanism of retrieving
location information from the mobile station can be defined in various ways. As
previously mentioned the mobile station can either broadcast its whereabouts at pre¬
defined intervals, or it can be polled by the service provider, or a combination of the
two methods may be used where the broadcast interval can be dynamically altered to
provide an adaptive system. This is particularly useful in built-up areas or where the
mobile station is perceived to have excessive velocity to warrant a higher broadcast
rate. By having control of the location updating procedure, the additional bandwidth
requirement can be minimised.
It would be possible for the mobile station to incorporate circuits capable of
calculating its own position from the signal strengths of the adjacent base stations but
generally the system will operate such that the mobile station transmits to the service
provider the identity of the six strongest signal strengths in its location. The service
provider would calculate the location from the received information.
The improved resolution achieved in accordance with the present invention is
clearly of value in all of the many circumstances in which it has been proposed to
obtain value from being aware of the location of a mobile station. The present
invention does however offer further opportunity given the fact that the invention
relies upon the widely available GSM technology. It would be possible for example
to provide a subsystem of a standard cellular telephone mobile station which did no
more than report to the system provider its location in appropriate circumstances.
Such a subsystem could be fitted in for example pieces of equipment which are liable
to be stolen, such as personal computers. The personal computer could be arranged to in effect broadcast its location as soon as it was disconnected from the mains supply.
This would mean not only that the personal computer would generate a warning signal
indicating that it was being tampered with, but in addition it would enable the police
to track its movement after it has been tampered with. Given that the necessary mobile station equipment can now be manufactured at a relatively trivial cost the
application of the present invention in this context has very significant implications in
the field of locating stolen property, particularly as the necessary equipment is very
small and therefore easy to conceal. The present invention therefore has the ability to
provide a function exactly equivalent to that used by the dedicated vehicle tracking
systems now on offer but at a far lower cost.
Although the mobile station location may be determined by reference to the
signal strengths received from a series of base stations, this location information can
be enhanced by reference to timing advance data. When a mobile station is
communicating with a base station, existing systems apply a timing advance to
maintain synchronisation. This makes it possible to determine with accuracy the
distance of the mobile station from the one base station with which it is
communicating. This information can be used in conjunction with the signal strength
and map data to improve resolution. Of course timing advance data is not available
when a mobile station is not communicating with any base station.
It should also be noted that the disposition of base stations makes it possible to
provide good resolution adjacent to many major transport routes. Generally, base
stations are located close to major routes such as motorways. The location of a
mobile station moving along a motorway can therefore often be very accurately determined from the signal strengths of the two base stations between which it is
located.
The resolution with which mobile station location can be achieved may also be
improved by reference to the process of "handover", that is when the primary cell with
which a mobile station communicates changes due to movement of the mobile station
between the cells. Given that the system operator can detect when handover occurs
and that that will indicate to the system operator that the mobile station is crossing a
reasonably well defined line defined between the two cells involved in the handover,
this information can be added to the other position information to improve accuracy.